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CHAPTER 11 – INSECT PESTS THE WHEAT BOOK MAJOR PESTS (continued) Cockchafers Damage Cockchafer larvae feed underground and some species are serious pests which may slow growth or kill large areas of cereals by eating plant roots. Young plants without extensive root systems are worst affected and more than three grubs per shovel full of soil will cause some damage while five or more may completely destroy the crop causing erosion problems in the subsequent summer. The adult beetles are also destructive as they feed on tree foliage. The cockchafer which has caused the most damage to wheat crops in Western Australia is Heteronyx obesus. Figure 11.9: Cockchafer larvae Description There are several important pest species and adults range in size from 5 to 20mm. Adults are usually brown or blackish beetles sometimes called “ Christmas beetles” . The beetles fly readily and are attracted to lights. Cockchafer larvae are characteristically “ C” shaped creamy white grubs between 2 and 25mm long. Life cycle The complete life cycle may take one or two years. Some species have a long larval stage which extends over 12 to 18 months. In most species the larva is active during late autumn and winter, then pupate in spring with adults emerging in early summer. Feeding, mating and egglaying may occur throughout summer. Control Control of cockchafer larvae is usually not economical. The main pest species in this State cannot be controlled with chemicals after planting as the larvae remain underground. It is not possible for insecticides to penetrate the soil far enough to have any effect; even following heavy rainfall. Shallow planting and planting with treated seed at a high rate may help to overcome the problem by establishing a strong root system early in the season. The problem is most serious when early growth is slow. Large populations of non-pest species may be present under 238 Figure 11.10: Cockchafer adult young crops and in pasture without causing visible damage. The organophosphate chlorpyrifos is registered for cockchafer control in cereals.
THE WHEAT BOOK CHAPTER 11 – INSECT PESTS MAJOR PESTS (continued) False wireworm Damage False wireworm larvae may be present in the soil in high numbers with little or no damage to plant seedlings because they are primarily feeding on dead organic matter. Occasionally, larvae do chew into the seedling stem and roots, weakening the plant or ring barking the stem and causing the plant to fall over. Strong winds, which buffet young fragile seedlings, increase the level of plant deaths. In swathed crops adult beetles may cause a problem through contamination of grain samples, as adults shelter within the swath and are then caught up into the harvester. Description False wireworm is a general description given to several species of soil dwelling beetle larvae belonging to the family Tenebrionidae. They are primarily scavengers, feeding on fungi and dead plant material. The larval or worm stage of the beetle’s life cycle has been responsible for attacking some seedling crops. Two main species are known to cause damage in Western Australia: the Vegetable beetle (Gonocephalum elderi) and the bronze field beetle (Adelium brevicorne). Figure 11.11: Bronze field beetle Control Increased stubble retention and use of minimum tillage, allowing an abundance of plant material on the surface, favours beetle survival and increase. False wireworms are not a regular pest of cereal crops and use of chemical control is often uneconomical. As it is difficult to anticipate false wireworm damage, the best option may be to observe the crop carefully and replant the affected parts of the paddock if necessary. The organophosphate Chlorpyrifos is the only chemical registered for control of false wireworm in cereals. 239
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